The invention described herein is an endoprosthesis for total knee arthroplasty (TKA), the surgical technique meant to reconstruct and rehabilitate the painful and deformed knee joint.
The motion of the normal knee joint is a continuous combination of rolling, sliding and rotation, valgus-varus angular displacement of the femoral condyles on the tibial articular surface and the menisci, which collectively is referred to as flexion-extension. These complex motion components take place in three dimensions and always in a varying combination, and are controlled by the shape of the articular surfaces, the articular capsule, the menisci and the ligaments.
Longterm and good quality function of a TKA endoprosthesis is influenced by and depends upon the manner adopted by the implant design to solve the following interrelated problems: the wear of materials, the fixation of the endoprosthesis and the satisfactory joint motion.
The femoral component of a TKA endoprosthesis resembles the distal part of the human femur and is either metallic or ceramic. The tibial component of a TKA endoprosthesis is metallic, having a flat upper surface and possessing at least one stem on the lower surface for fixation onto the tibial bone. Between the femoral and tibial components another one is inserted: the polyethylene insert, which may be either fixed or mobile on the flat upper surface of the tibial component.
When the polyethylene insert is mobile, its motion is controlled by spigots protruding from the upper surface of the tibial component and corresponding recesses on the lower surface of the polyethylene insert, or vice versa. This control system may allow rotation or also simultaneously anteroposterior translation.
During TKA surgery, the articular cartilage and the subchondral bone are removed from the distal femur and the proximal tibia. Using appropriate instrumentation, these anatomical areas are contoured in order to allow the exact fitting of the endoprosthetic components, with or without use of acrylic resin cement. The anterior cruciate ligament is most of the times removed, while the posterior cruciate ligament is either preserved or sacrificed and substituted by functional characteristics of the implants. Special surgical attention is being paid in the preservation of the collateral ligaments of the knee and the symmetry of the ligamentous tensions developed during joint motion.
From the above description, it can be appreciated that with a given competent surgical technique, the longterm good quality function of a TKA endoprosthesis lies with the correct design of its individual components; this latter condition being perceived in a particular biomechanical sense reflecting the kinematic compatibility between the pre-existing musculoskeletal flexion-extension mechanism of the patient and the corresponding mechanism to which the individual components obey by-design.
Successive design generations attempted to fulfil the above condition, initially using fixed polyethylene inserts and later on mobile ones; the latter introduced in order to better address the issue of material wear and longevity. Mobile polyethylene inserts in particular, were introduced based on the rationale that, while the endoprosthesis should have the required congruent articular surfaces within the full range of joint motion and independently from the tibial rotation with respect to the femur, on the other hand it (the endoprosthesis) should direct a large amount of the developed loading towards the surrounding soft tissues (capsule, ligaments, tendons-muscles), thus protecting the polyethylene as well as the fixation of the implants onto the bones.
Today, when correctly implanted, all commercially available TKA endoprostheses generally demonstrate good clinical results, with a survival percentage of 95% at ten years from surgery.
However, all commercially available TKA endoprostheses present today the following disadvantages:
a) They demonstrate limited maximum flexion angle (ranging between 115 to 127 degrees), which is not compatible with activities of younger patients as well as with daily activities of populations needing to flex their knee joints above 150 degrees, and
b) When driven to higher knee flexion angles, they present an extremely reduced area of load bearing articular surface between polyethylene and femoral condyles, which results in faster material wear and loss of implants fixation onto the bones.